1. Field of the Invention
The present invention relates to a vapor deposition apparatus adopted for manufacturing compound semiconductor thin films on substrates, and particularly to a vapor deposition apparatus which smoothly rotates and revolves substrates to form thin films having uniform thickness distribution on the substrates.
2. Description of the Prior Art
There are several conventional vapor deposition apparatuses which are generally used to manufacture heterostructure compound semiconductors. Among such conventional apparatuses, FIGS. 1, 3 and 4 show vertical-type vapor deposition apparatuses, and FIG. 2 shows a horizontal-type vapor deposition apparatus.
In FIG. 1, a revolving turntable 5 is disposed in a silica reactor tube 1 and rotated by a motor 6. On the turntable 5, there are disposed a plurality of susceptors 4 made of graphite material, etc. A rack 4a formed on the periphery of each of the susceptors 4 engages with a gear 9. The gear 9 is rotated by a motor 7 to rotate the susceptors 4 and a plurality of substrates 3 respectively held by the susceptors 4. The susceptors 4 are heated by a high-frequency coil 2 disposed around the silica reactor tube 1 to heat the substrates.
While the susceptors 4 are being rotated and revolved with the temperature of the substrates 3 being maintained at a predetermined value, reactive gases, for instance organometallic gases such as (CH.sub.3).sub.3 Ga, (CH.sub.3).sub.3 Al and AsH.sub.3, are alternately or simultaneously supplied from a gas supplying port 10 to grow crystals on the substrates 3 which are maintained at the high temperature.
In such a vertical-type vapor deposition apparatus of the prior art, the susceptors 4 are generally heated to about 700.degree. C. to 800.degree. C. to grow the crystals. As a result, if the quantity of each reactive gas is small, heat convections are caused on the substrates 3 which generate vortexes that disturb the flows of the gases. If the flows are disturbed due to these vortexes, the surfaces for growing crystals will not provide heterostructures, leading to a deterioration in the electrical characteristics of the thin films to be formed on the substrates 3. Particularly, in growing ultrathin films, each having a crystal film thickness of several 10 .ANG., the reactive gases would be quickly changed from one to another, thereby causing serious problems due to heat convections.
In the horizontal-type vapor deposition apparatus shown in FIG. 2, a revolving turntable 15 is disposed at the bottom of a reactor tube 11 and rotated by a motor 16. On the turntable 15 there are disposed a plurality of susceptors 14 each holding and rotating a substrate 13. A rack 14a is formed on the periphery of each of the susceptors 14 and engages with a gear 19. The gear 19 is rotated by a motor 17 to rotate the susceptors 14. The susceptors 14 are heated to a predetermined temperature by an infrared lamp 12 disposed outside the reactor tube 11, thereby increasing the temperature of the substrates 13 to a predetermined value. After that, reactive gases are supplied from a gas supplying port 20 in the same manner as described in the above to react and grow crystals on the substrates 13.
In this horizontal-type vapor deposition apparatus, the reactive gases flow horizontally to make a laminar flow such that the formation of vortexes are not so significant and the disturbance in the flow is relatively small. However, with respect to the gas supplying port 20, there is a large difference in the distance between a proximal portion and a distal portion of each of the substrates 13. Therefore, the crystal growing rates of the proximal and distal portions of the substrate 13 differ from each other, resulting in unevenness in the thickness distribution of the film formed on the substrate 13. Particularly, in forming an ultrathin film on the substrate, this problem is not solved even if the substrate is rotated and revolved because there are limits to the rotating and revolving speeds of the substrate and because reactive gases, each in small quantities, are changed from one to another in short time intervals before the substrate 13 can complete a single revolution. Namely, the reactive gases are changed from one to another at a more rapid rate than that of the rate of revolution for the substrate 13. As a result, differences in the composition ratio and thickness distribution of a film to be formed on the substrate are increased.
Since the area of the turntable 15 is large, the flows of the reactive gases are more disturbed as the revolving speed of the turntable increases. In mass production, the number of substrates 13 is large, so the area of the turntable 15 needs to be increased, thus increasing the size of the vapor deposition apparatus.
FIGS. 3 and 4 are a general perspective view and a cross-sectional view, respectively, showing another vertical-type vapor deposition apparatus.
In the figures, a shaft 22 for supplying reactive gases is disposed substantially at the center of a reactor tube 21. A susceptor 23 is supported by a plurality of bases 24 arranged around the shaft 22. The bases 24 are fitted to a hub 25 which is rotated by a motor (not shown) to rotate the susceptor 23. On an outer surface of the susceptor 23, there are rotatably disposed susceptor rotors 26 on each of which a substrate 27 is mounted. The periphery of each susceptor rotor 26 frictionally makes contact with a fixed track 28. When the susceptor 23 is rotated, the susceptor rotors 26 are also rotated with respect to the fixed track 28. As a result, the substrates 27 are simultaneously rotated and revolved.
One feature of the vapor deposition apparatuses shown in FIGS. 1 through 4 is the simultaneous rotation and revolution of substrates. However, a common problem was found to exist in the above-mentioned prior arts, according to experiments carried out by the inventors, in that products of reaction adhere to the substrate rotating mechanisms (e.g., the gears 9 and 19, the racks 4a and 14a and the motors 6 and 16 of the apparatuses shown in FIGS. 1 and 2, and the peripheries of the susceptor rotors 26 and the contacting portion of the fixed track 28 of the apparatus shown in FIGS. 3 and 4) exposed to the reactive gases in the reactor tubes, thereby hindering the smooth rotation and revolution of the substrates as time elapses, and thus causing unevenness in the thickness of the film formed on each substrate.
In the apparatus shown in FIGS. 3 and 4, when the susceptor 23 is inductively heated with a high-frequency coil, an induction current flows through the susceptor 23 which is a conductor, and electrical discharge phenomena are caused between the susceptor 23 and the susceptor rotors 26 due to sliding motions between them. As a result, the contacting faces of the susceptor 23 and the susceptor rotors 26 are quickly worn, leading to a deterioration in the durability of the apparatus and thus increasing the need for frequent maintenance. In addition, due to the electrical discharges, high-frequency outputs vary and this destabilizes the temperature of the susceptor 23, which subsequently deteriorates the uniformity of the crystalline thin film formed on each substrate 27.